1. Field of the Invention
This invention relates generally to ion implantation apparatus used to decelerate ion beams to sub-keV energies. Specifically, this invention relates to an improved deceleration optics system that functions as an energy-filter to allow only charged particles within a particular range of energies to reach the targeted wafers.
2. Background
Ion implantation is a ballastic process used to introduce atoms or molecules, called dopants, to make materials with useful properties. In particular, ion implantation is a common process used in making modern integrated circuits. The amount of ion beam current that can be transported in a conventional ion implanter depends on the ion beam energy and at low energies this beam current becomes unfeasibly low.
For a conventional high current ion implanter, an ion beam is extracted from an ion source and travels through a mass analyzer to select specific ionic species. The selected or filtered ion beam emerges from the magnet and is then incident toward the semiconductor target wafers. The travel distance from the source to the wafers is usually about two meters. For an ion beam with an energy as low as 0.2 keV and beam currents as high as 10 mA, the space charge of the beam is so intense that the ion beam starts to blow up severely as it leaves the source. This problem exists regardless of what kind of beam focusing optics is used. After the ion beam travels about two meters there is not much usable beam current left for implantation. An efficient way to obtain high beam currents at low ion energy is to decelerate an ion beam from higher energy, e.g., 5 keV, to a lower energy, e.g., 1,0.5, or as low as 0.2 keV, at a region close to the wafers. Although the beam may also blow up after deceleration, there is still sufficient beam current remaining for implantation because the distance between the deceleration region and the wafers is usually less than 0.4 meters. With the use of a plasma or electron shower, the beam blow-up will be less and beam transmission can be improved.
The above method is able to achieve high beam currents at energies below 5 keV by extracting ions at a higher than desired final energy, conducting a mass analysis of the ions, and then decelerating the ion beam to the desired energy just before it reaches the target. However, high-energy neutrals can be generated in the region between the mass analyzer and the deceleration electrodes when higher energy ions have charge exchange interactions with residual gases in the beamline. These neutralized atoms will not be decelerated by the decelerating electric fields and will reach the wafers at higher than desired energies. This results in what is known as energy contamination, which causes a deeper than desired dopant depth profile. Energy contamination is only tolerable to xcx9c0.1% in order to provide sufficient margin against shifts in device performance [L. Rubin, and W. Morris, xe2x80x9cEffects of Beam Energy Purity on Junction Depths in Sub-micron Devicesxe2x80x9d, Proceedings of International Conference on Ion Implantation Technology, 1996, p96]. To have such a low neutral fraction it requires that the chamber pressures be kept very low (5.0E-7 torr) so as to minimize the probability of charge exchange reactions. This level of pressure is, however, very difficult to maintain under normal operating conditions in an implantation system due to the out-gassing of the photo-resist coating of patterned devices and the presence of feed gases from the source and plasma shower. Another issue is the variation in the level of contamination. Pressure fluctuations during the implant can cause across wafer effects. Day-to-day changes in residual vacuum or photo-resist quality can cause batch-to-batch effects. Finally, the potential loss of wafers worth millions of dollars exists due to these types of undetected vacuum problems.
For all of the above reasons, traditional techniques of ion implantation using conventional deceleration approaches as described above do not provide a viable solution for very low energy ion implantation. There is a need in the art of IC device fabrication to provide new systems to provide very low energy implants with minimal energy contamination. For devices that require shallow p-type and n-type junctions new methods and systems are required to resolve the difficulties and limitations of low energy ion implantation with effective control over energy contamination.
It is the object of the present invention to provide a new ion implant system for very low energy (sub 2 keV) implants to form shallow p-type and n-type junctions in semiconductor devices. The new ion implant system has novel deceleration optics that will enable those of ordinary skill in the art to overcome the problems encountered in the prior art.
Specifically, it is the object of the present invention to present a new ion beam steering deceleration and steering system for decelerating a charged ion beam and for separating the neutralized component, or neutral fraction, from the main ion beam. The charged ion beam is filtered and focused by the ion beam deceleration optics and becomes an angularly spread out beam with an angle of deflection that is dependent on the ion energy. In this way, the ion implant energy can be more accurately controlled and the neutral fraction can be removed. The neutral beam is unaffected by the decelerating electric fields and propagates in the same direction as the initial beam before deceleration. A neutralized-particle stop block then stops the neutrals before reaching the target wafer or target chamber. Energy contamination resulting from neutralized particles incident to the target with higher than desired energies is thus resolved.
The electrodes of the beam deceleration optics are configured to move in a traverse direction relative to the beam line so that the beam can be steered to travel further away from both the neutralized and high-energy particles to assure that only low energy ions are employed for implantation.
Briefly, in a preferred embodiment, the present invention discloses an ion source apparatus for generating and directing an ion beam. The ion source apparatus includes a beam deceleration optics for decelerating and filtering the ion beam. The beam deceleration optics includes a plurality of electrodes for generating an electrostatic field for filtering and spreading out the ion beam over an angular range according to the energy of each ion for more accurately directing a low energy ion to a target wafer.
More specifically, an ion implantation apparatus is disclosed in this invention that includes a target chamber for containing a target for implantation and an ion source chamber that includes an ion source with a mass analyzer for generating an ion beam with specific mass at original energy. The ion source chamber further includes beam deceleration optics for decelerating the ion beam from the original energy to the desired final energy. The ion beam apparatus is able to accurately direct low energy ions to a target wafer. The beam deceleration optics further includes a plurality of electrodes for generating an electric field for spreading the charged ion beam over an angular range to accurately control the trajectory paths of ions of different energy levels. The purpose is to eliminate the energy contamination by more accurately controlling the energy range of the charged ions that reach the target and to block the neutralized particles and ions with higher energy from reaching the target
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures.